This invention relates generally to methods and apparatus for cardiac CT imaging, and more particularly to methods and apparatus that minimize an impact of heart motion in collecting calcification data from coronary images.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two-dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
A main objective of cardiac CT applications is to perform calcification scoring, a diagnostic procedure in which an amount of calcification present in a patient's heart is estimated. At least one known CT imaging system requires about 0.5 s to complete data acquisition for an image. Although this speed is satisfactory for general imaging purposes, it is not fast enough to avoid motion-induced image artifacts in cardiac CT imaging, in which a typical cardiac cycle is about 1.0 s long. These artifacts present major problems for cardiac calcification scoring.
At least one other known CT imaging system reduces motion-induced image artifacts by acquiring data rapidly enough to effectively freeze cardiac motion. This imaging system employs a scanning electron beam to generate a moving source of x-rays rather than an x-ray source and detector on a rotating gantry. However, CT imaging systems employing scanning electron beams are quite expensive and are not available at many hospitals.
It would therefore be desirable to provide methods and apparatus that overcome motion-induced artifacts produced in images acquired by CT imaging systems having relatively slow scanning and detection systems such as rotating gantries. It would also be desirable to provide cardiac calcification scoring methods and apparatus utilizing such CT imaging systems. It would further be desirable to provide methods and apparatus that can readily identify and score calcification from the small incremental x-ray attenuation produced by small amounts of calcification.